Process for treating synthetic ester lubricants with aromatic isocyanates or aromatic thioisocyanates



United States Patent PROCESS FOR TREATING SYNTHETIC ESTER LUBRICANTS WITH AROMATIC ISOCYANATES 0R AROMATIC THIOISOCYANATES Dietrich F. Huttenlocher, Philadelphia, Pa., and William M. Sweeney, Wappingers Falls, N.Y., assignors' to Texaco Inc., New York, N.Y., a corporation of Delaware No Drawing. Filed Nov. 21, 1960, Ser. No. 70,370

6 Claims. (Cl. 260404.8)

This invention relates to a method of treating synthetic ester lubricating oils. More particularly it relates to a novel method of reducing the acidity and hydroxyl content of synthetic ester oils employing aromatic hydrocarbyl isocyanate or an aromatic hydrocarbyl isothiocyanate as the treating agent.

Synthetic esters are widely used as lubricating oils in engines which are subject to high temperatures such as combustion turbine engines, particularly those of the turbo-jet and prop-jet type. The reason for this is these synthetic ester oils are generally characterized by a higher viscosity index and lower pour point than mineral oils of corresponding viscosity. In addition they leave a lower percentage of carbonaceous residue than their mineral oil counterparts. carbonaceous residue from engine oils is undesirable in that it accumulates and interferes with the operation of the engine. In addition the synthetic ester lubricants generally have higher flash points than mineral oils of equivalent viscosity thereby lessening the danger of combustion of the oil at high engine temperature operation.

The synthetic esters can be broadly described as esters of hydrocarbyl carboxylic acids and hydrocarbyl alcohols. They are high molecular weight materials of lubricating oil characteristics derived from alcohols which are usual- 'ly aliphatic alcohols containing from 1 to 4 or more hydroxyl radicals and carboxylic acids which are usually aliphatic carboxylic acids containing from 1 to 4 or more carboxyl acid radicals.

Widely used synthetic ester lubricants are aliphatic esters of saturated aliphatic monocarboxylic and dicarboxylic acids containing 6-l2 carbon atoms. From the standpoint of cost and availability the preferred carboxylic acids are caproic, heptoic, caprylic, pelargonic, nonylic, adipic, sebacic and azelaic acids. The monohydric and polyhydric aliphatic alcohols used to form the esters usually contain at least 4 carbon atoms and up to 20 or more carbon atoms. C to C saturated aliphatic alchols are commonly used such as Z-ethylhexanol and 2,2-diethyl-1,3-propanediol. Ether alcohols such as 2 ethoxyethanol and monoethyl ether of diethylene glycol may also be employed in the formulation of ester oils. Alcohols containing two or more hydroxyl radicals and an absence of hydrogen on the beta carbon atom such as trimethylolethane, pentaerythritol and trimethylolpropane have proven particularly eifective in formulating stable high temperature ester lubricants.

Examples of simple esters of a aliphatic carboxylic acid and alcohol are the following: diisooctyl azelate, di-2 ethylhexyl sebacate, di-Z-ethylhexyl azelate, di-Z-ethylhexyl adipate, dilauryl azelate, di-sec-amyl sebacate, di- 2-ethylhexyl alkenyl succinate, di-Z-ethoxyethyl sebacate, di 2-(2'-methoxyethoxy)ethyl sebacate, di-2-(2-but0xy ethoxy) ethyl sebacate, di-Z-butoxyethyl azelate, di-2-(2- butoxyethoxy) ethyl alkenylsuccinate, tetracaproate of pentaerythn'tol, triisoctanoate of trimethylolpropane and nonyl caproate.

In addition to such esters, complex ester lubricants formed by a reaction of an aliphatic polycarboxylic acid, an aliphatic polyhydroxy compound and a monofunctional aliphatic alcohol or acid are also employed as synthetic lubricants. Complex esters of this type are described in US. 2,628,974. Complex esters formed by the reaction of a mixture containing stoichiometric amounts of 2-ethyl-1,3-hexanediol, sebacic acid, and 2- ethylhexanol and by reaction of a mixture containing adipic acid, diethylene glycol, and Z-ethylhexanoic acid illustrate this class of synthetic complex ester lubricating oils. Further illustrations of the complex esters are the ester reaction products of sebacic acid, trimethylolpropane and pelargonic acid; sebacic acid, trimethylolpropane, and 2-ethylhexanol; sebacic acid, trimethylolethane and hexanoic acid; and'suberic acid, 1,3,5,7-octanetetraol and pentanoic acid.

The sulfur analogues of the foregoing esters are also useful as synthetic lubricants. For example, simple thioesters are illustrated by di-Z-ethylhexyl thiosebacate, din-octyl thioadipate and the dilaurate of 1,5-pentanedithiol; sulfur analogues of complex esters are exemplified by the reaction product of adipic acid, thioglycol and 2- ethylhexyl mercaptan.

In the preparation of the ester lubricating oils manufacturers are faced with the problem of lowering the final acidity and hydroxyl content of the oil to a point where the synthetic ester lubricant will be acceptable for use in engines at high temperatures. The acidity and hydroxyl content of the synthetic ester lube oil are derived from carboxyl (COOH) and hydroxyl (OH) present therein. These radicals are generally always present in crude synthetic ester oils even though in the preparation of the synthetic ester lube the amounts of acid and alcohol reactants are normally proportioned in such a manner that there is equal or near equal number of carboxyl and hydroxyl groups present for reaction with one another. The reasons for the residual amount of free carboxyl and hydroxyl radicals which cause the acidity and hydroxyl content of the synthetic ester lube oil are believed to be three-fold. Firstly, the proportions of alcohol and acid reactants employed may not give an equal number of carboxyl and hydroxyl groups, thereby promoting an excess of one or the other in the ester oil product. Secondly, particularly in commercial preparation of ester lube oils, it is uneconomical from a time standpoint to allow the esterification reaction to go to completion, thus small quantities of unreacted alcohol and acid. as well as partially esterified acid and alcohol molecules remain in the final product. Thirdly, although the water by-product is normally continuously removed in ester oil formation, 100% removal is not economically feasible. This residual water causes a small part of the ester oil to hydrolyze to the initial alcohol and acid reactants and/or into partial esters.

The acidity of the synthetic ester lube is detrimental in that it promotes corrosiveness of the lube towards metals. Furthermore, a hydroxyl content in the lube is undesirable in that it enhances the thermal and oxidation instability of the lube.

In the past the residual acidity of ester oils has been reduced by washing with aqueous alkali or by percolating through beds of alkali or alkali impregnated on less active materials. Washing with alkalies, however, frequently results in the formation of stable aqueous emulsions. Although the percolation method may reduce the acidity to the desired point, ester soluble alkali salts of organic acids apparently result since the ester lubricants so treated have been found to possess a high and undesirable ash content. Still further the percolation method has not been found economically desirable since it can result in loss of ester product of approximately 40 to 50%. It has been further proposed to eliminate the acidity of the synthetic ester lubes by using excess alcohol in their preparation. However, this results in a synthetic under reduced pressure.

. efiective for reducing hydroxyl content and also caused.

a lube. of asignificant hydroxyl content which tends to render it thermally unstable and sensitive to oxidation.

' In respect to hydroxyl content it has been proposed to remove 1 the hydroxyl bearing ingredients in a synthetic ester'oil by subjecting the esterto fractional distillation This procedure was not found the "ester oil to undesirably darken. The darkening eifect is a result of the partial decomposition of the ester oil promoted by the hydroxyl groups therein. a I

We have discovered and this forms the object of the invention that the acidity and hydroxyl content of crude synthetic ester lubricatingoils may be substantially reduced by treatment withanaromatic 'hydrocarbyl isocyanate or aromatic hydrocarbylisothiocyanate We have further found this treatment does not produce theundesirableeffects of past methods such as high product loss, femulsion formation and discoloration of the ester oil.

The treatment is' accomplished by reacting the synthetic ester lube with'the isocyanate or isothiocyanates at an elevated temperature, e.g. between about 50 and 250 C. The amounts of aromatic .hydrocarbyl isocyanate or iso-' thiocyanate employed to reduce the residual acidity and ly low acidity but has a high hydroxyl content;

thiocyanate in the manner previously described.

In order to further-illustrate, the method of the present. invention the. following examples are given. It is mbeunderstood that these examples are.illustrative-only-and' are not considered to-be limiting inany. wayupon the concept of this invention; 7 a

Examplel In a 5-liter 3- neck flask equipped with .a mechanical stirrer, a thermometer and a 'condenser134 grams (1.0

. mole) of trimethylol ropane,;13l grams (0.65 mole) of hydroxyl content of the c'rudeester willof course depend on the quantity. of ester and the quantity of carboxylicand hydroxyl groups therein. However, it has been found that amounts of .isocyanate or isothiocyanate between about 0.1 and 10%, preferably between-1 and 5% based on the weight of theesteroil are generally sufiicient'to reduce neutralization. number (Neut; No.) and hydroxyl number (OI-l No.) of the ester oils prepared by any of the N0, and OH No are respectivelyrelativenumerical rep resentations of the carboxyl and'hydroxyl concentration in a' material. V

Under the aforementioned operating conditions. the.

Neut. No. and OH No. ofthe ester oil canbe significantly reduced in a-period between about 1 and 20 hrs.

In'orderto facilitate the reaction of the isocyanate or. isothiocyanate with the ester, an inert liquid organic dilu-.

entsuch as toluene and xylene may be employed. Furstandard methods to desired levels, eg, to a Neut. No. less than 1 and an OH No. less than 2. It is to be notedNeut;

thermore, under preferredreaction. conditions the. ester oil and. isocyanate or isothiocyanate are continuously mixed under a blanket ofinert gas'such as nitrogen. The.

purposeof the inert gas is to insure against possible pre: 7

mature oxidation of. the isocyanate or isothiocyanate re actant. 7 i a i Specific examples of the aromatic hydrocarbyl isocyanatesand isothiocyanates contemplated herein are the aryl, alkaryl and aralkyl isocyanates and isothiocyanates of 7 to 30, carbon atoms such as phenyl isocyanate, phenyl. isothiocyan'ate, 4-tolyl. isocyanate, 4-.tolyl .isothiocyanate,

' 3-phenylbutyl isocyanate, benzyl 'isocyanate, and benzyl isothiocyanate. p

The reactionof the isocyanate or isothiocyanate with the synthetic ester lubricant to reduce its acidity and hydroxyl content has beentheorized in the following man net. It is believed that the isocyanate reacts with the carboxyl groups in. the synthetic ester to form an amide linkage. In respect to'the .hydroxyl groups the isocyanate V reacts to form a urethane structure. Still furtherthe isocyanate is believed to form asymmetrically disubstituted urea with unremovedwater produced in the esterification reaction forming the ester oil. The urethane, amide, and

-urea productsfformed in the treatment appear to havev some beneficial effect on the properties of the synthetic oil and need not be removed therefrom. However, it is desirable to separate anyprecipitated products from the ester oil such as by filtration or vacuum stripping.

Ina modified procedure of our novel method a stoichiometric excess of alcohol is employed, for example, .between about 1 to 10 wt. percent in excess of the amount necessary for the complete esterification of the acid. The crude ester produced under these conditions is of relativesebacic acid, 268 grams (1.7 mole) of pelargcnic acid and 50 milliliters of xylene were refluxed for-.24 hours,at 250 C; During the reaction period 54 -n1ls.'ofwater-resulting, froni the esterification were removed. Analysis "of the f, crude ester product found it to have anOH No. 1(hydroxylf number) of 10. V

. 500 gramsofathis ester product were admixed with 5 grames of phenyl isocyanate and were heated for thours; at 1'70 to19.0 'C. The thus treatedester product was ana:

lyzed andfound to have an.OH No. of 0;

' Example 2 i To a 12-liter 3-neckflask equipped with a stirrergtwo water separators and condensers, 1340 (10 moles) grams trimethylolpropane, 1313 grams (6.5 moles). sebacic acid. v 2636 grams (17.0 moles) pelarg onic' acid,and 300mil-' liliters of xylene were added, Thereaction mixture was. refluxed for.22 hours at. a temperature of 255-275 C. Atthe end of the reaction period 535 millilitersof water; or over 99% of the theoretical amount of .water resu1ting. from complete e'sterification was removed. The Neut. No.

of'the crude ester product was 3.5.

The ester product was cooled. to 200 C. and while bubblinglnitrogen through the product to forma blanket. of nitrogen thereover, SSIgrams of phenyl isocyanate-was added over a period of 5 minutes. The ester productandl isocyanate were held at said temperature with. mixing for a period of 6 hours. 1 At the end of the 6hourperiodj the thus treated ester was analyzed, found to have a Neut.

No. of 0.32. The treated ester (.wf. 4923' grams)fwas filtered to remove small amounts of. crystalline diphenylurea and was stripped to 230. Cat. 15 mm. Hg pr e ssur r for one hour. The stripped ester'weighedv 4505 grams (94.5% molar yield) and had a Neut. No. of 0.10,'a kinematic viscosity at 210;F. -of 47.2 cs. and a pour point of minus 25 F. n 7

Example 3.

The esterificationand phenyl isocyanate treatment procedures of Example 2" wererepeated except; 1366' grams (1 0.2 moles) of trimethylolpropane, 13'13grams;

(6.5 moles) sebacic acid and 2686 grams 17.0 moles pelargonic acid were employed'therebyforming an esterifi-. cation' reaction mixture containing a 2 mole percent alcohol excess on an equivalent basis over the totalfacid,

In addition 54 grams. of phenyl isocyanate;

reactants. V were employed. At the end of the ester-forming reaction period the crude synthetic lubricant had ajNeut. No. .of 1.8. Afterthe phenyl isocyanate treatment;the stripped product had a Neut. No. of 0.1. 1

Example 4 To a 12-liter, 3-neck flask equipped with a stirrer, a. thermometer and a dropping funnel therewere'charged 8770 grams of a complex ester of a Neut'. No. of 3.7'result-; ing from the reaction of trimethylolpropane, pelargonic acid and sebacic acid. Nitrogen was bubbled through the material which was heated with stirring. .When the pot temperature'reached- 200 C.,. grams of phenyl isocyanate were added 'dropwise through the dropping fun- Heating was continued for 12 hrs. at 190'200'C.

nel. At the end of the 12 hour period the ester was analyzed and found to have a Neut. No.. of 0.8. Subsequently This high hydroxyl containing ester is treated with isocyanate or iso-:

the isocyanate treated ester was stripped at 230 C.l mm. Hg and crystalline diphenylurea formed during the isocyanate treatment Was removed by filtration. The Neut. No. of this finished ester product was 0.3.

We claim:

1. A method for reducing the acidity and hydroxyl content of a synthetic ester lubricating oil derived from the esterification of a hydrocarbyl carboxylic acid with a hydrocarbyl alcohol and containing unesterified substituent groups derived from said esterification, said substituent groups selected from the group consisting of unesterified free carboxylic acid groups, unesterified tree alcohol groups and mixtures thereof comprising contacting said synthetic ester lubricating oil containing said unesterified free substituent groups with an aromatic compound of 7 to 30 carbons selected from the group consisting of phenyl monoisocyanate, phenylalkyl monoisocyanate, alkylphenyl monoisocyanate, phenyl monoisothiocyanate, phenylalkyl monoisothiocyanate, and alkylphenyl monoisothiocyanate at a temperature between about 50 and 250 C. so as to react said aromatic compound with said unesterified free substituent groups, said aromatic compound being present in a proportion between about 0.1 and 10 Wt. percent of said ester lubricating oil.

2. A method for reducing the acidity and hydroxyl content of a synthetic ester lubricating oil derived from the esterification of hydrocarbyl alcohol with hydrocarbyl carboxylic acid and containing unesterified free substituent groups derived from said esterification, said substituent groups selected from the group consisting of unesterified free carboxylic acid groups and unesterified free alcohol groups and mixtures thereof comprising contacting said ester lubricating oil containing said unesterified free substituent groups in an atmosphere of inert gas with from 1 to 5 wt. percent phenyl monoisocyanate based on the weight of said lubricating oil at a temperature between about 50 and 250 C. so as to react said aromatic compound with said unesterified free substituent groups.

3. A method for reducing the acidity and hydroxyl content of a synthetic ester lubricating oil derived from the esterification of hydrocarbyl carboxylic acid with hydrocarbyl alcohol and containing unesterified free substituent groups derived from said esterification, said substituent groups selected from the group consisting of unesteriiied free carboxylic acid groups and unesterified free alcohol groups and mixtures thereof wherein said oil was formed with between 1 and 10 wt. percent stoichiometric excess of said hydrocarbyl alcohol comprising contacting said ester lubricating oil containing said unesterified free substituent groups with an aromatic compound of 7 to 30 carbons selected from the group consisting of phenyl monoisocyanate, phenylalkyl monoisocyanate, alkylphenyl monoisocyanate, phenyl monoisothiocyanate, phenylalkyl monoisothiocyanate, and alkylphenyl monoisothiocyanate at a temperature between about and 250 C. so as to react said aromatic compound with said unesterified free substituent groups, said aromatic compound being present in a proportion of 0.1 and 10 wt. percent based on said oil.

4. A method in accordance with claim 2 wherein said ester lubricating oil is a complex ester lubricating oil resulting from the reaction of trimethylolpropane, sebacic acid and pelargonic acid, and said inert gas is nitrogen.

5. A method in accordance with claim 2 wherein said ester lubricating oil is a complex ester lubricating oil resulting from reaction of trimethylolpropane, sebacic acid and Z-ethylhexanol and said inert gas is nitrogen.

6. A method in accordance with claim 3 wherein said ester lubricating oil is a complex ester lubricating oil resulting from the reaction of trimethylolpropane, sebacic acid and pelargonic acid.

References Cited by the Examiner UNITED STATES PATENTS 2,861,981 11/58 Frank et al. 260485 r 3,053,871 9/62 Aries 260-448 FOREIGN PATENTS 738,254 7/43 Germany.

CHARLES E. PARKER, Primary Examiner.

T. E. LEVOW, L. ZITVER, Examiners. 

1. A METHOD FOR REDUCING THE ACIDITY AND HYDROXYL CONTENT OF A SYNTHETIC ESTER LUBRICATING OIL DERIVED FROM THE ESTERIFICATION OF A HYDROCARBYL CARBOXYLIC ACID WITH A HYDROCARBY ALCOHOL AND CONTAINING UNESTERIFIED SUBSTITUENT GROUPS DERIVED FROM SAID ESTERIFICATION, SAID SUBSTITUENT GROUPS SELECTED FROM THE GROUP CONSISTING OF UNESTERIFIED FREE CARBOXYLIC ACID GROUPS, UNESTERIFIED FREE ALCOHOL GROUPS AND MIXTURES THEREOF COMPRISING CONTACTING SAID SYNTHETIC ESTER LUBRICATING OIL CONTAINING SAID UNESTERIFIED FREE SUBSTITUENT GROUPS WITH AN AROMATIC COMPOUND OF 7 TO 30 CARBONS SELECTED FROM THE GROUP CONSISTING OF PHENYL MONOISOCYANATE, PHENYLALKYL MONOISOCYANATE, ALKYLPHENYL MONOISOCYANATE, PHENYL MONOIOSTHIOCYANATE, PHENYLALKYL MONOISOTHIOCYANATE, AND ALKYLPHENYL MONOISOTHIOCYANATE AT A TEMPERATURE BETWEEN ABOUT 50 AND 250*C. SO AS TO REACT SAID AROMATIC COMPOUND WITH SAID UNESTERIFIED FREE SUBSTITUENT GROUPS, SAID AROMATIC COMPOUND BEING PRESENT IN A PROPORTION BETWEEN ABOUT 0.1 AND 10 WT. PERCENT OF SAID ESTER LUBRICATING OIL. 